1. Field of the Invention
This invention relates to cleaning of surface well equipment and it more particularly relates to the complete removal of drilling mud, other solid and oil, from the surface well equipment prior to introduction of the solids-free brines or other well fluids having an affinity for solids.
2. Description of the Prior Art
There are certain completion and packer fluids, e.g., high density brines, that have a great affinity for entraining solids, which solids may originate in drilling muds or from drilling operations.
Aqueous brine solutions of high density are used in wellbores employed in the production of petroleum. These solutions have been used as both drilling, completion and packer fluids especially in deep wells subject to high formation gas pressures at elevated temperatures. These brines can be formed of the sodium, calcium, zinc salts with chloride, bromide and potassium. These aqueous fluids may include corrosion inhibitors and other salts such as soda ash. The density of these salt type well fluids depends on the particular salt, or mixture of salts, and their concentration in the aqueous well fluid. Usually, these salt type well fluids have a density in the range of between about 8 and 17 pounds per gallon.
The salt type well fluid should be solids-free in its use as a well fluid. If there are solids in a packer or completion fluid, they can cause serious injury to a producing formation by plugging of the pore spaces therein or even of the perforations and channels provided to induce fluid flows between the formation and well bore. If there are solids in a packer fluid, the solids will precipitate with time upon the packer. As a result, these solid deposits make it difficult to disconnect the tubing from the packer with a resultant costly well workover.
The high density brine can be prepared at the wellsite by dissolving the prescribed amount of salt into the aqueous phase, which phase is principally fresh or sea water but it can include various inhibitors for preventing pitting, corrosion, etc. The mixture is circulated or agitated in the surface well equipment until there are no undissolved salt solids. Naturally, the problems of adding salts to be dissolved in the aqueous well fluid became progressively more severe as the density increases, both in time, manpower and equipment requirements. Importantly, the surface well equipment must be scrupulously clean of any solids, especially the solid and oil portions of drilling mud. Otherwise, the prepared brine must be filtered to remove solids until the solids content has been reduced to less than 2-3 p.p.m.
At present, vendors will deliver to the wellsite the prepared high density brine of a desired density and combination of selected ingredients. The delivery of brine usually requires several changes in containers. For example, the brine is moved from the vendor tanks to truck transport, offshore supply boat and into the rig mud system. The brine can also become contaminated in the surface well equipment by entrainment of undesired solids, including residual water wetted solids and/or oil based drilling mud, weighing agents such as barite, rust, salt, silt and sand, and other undissolved materials. Contaminating liquids such as mud bases, lubricants and diesel fuel can also be present and entrained in the brine. The brine has a great affinity to entrain solids.
In selecting proper vendors, solids-free high density brine can be delivered to the well site, especially to offshore platforms. Attempts have been made to clean surface well equipment so as to prevent contamination of this brine with drilling mud and other resident solids.
It has been a practice to clean mechanically the rig's mud system of residual drilling mud by various washing and manual clean-up techniques. For example, offshore rigs use jet streams of sea water and crewpersons with mechanical scrapers, brooms, etc. in an attempt to remove residual drilling mud constituents. This technique for cleaning thoroughly the rig's surface mud equipment is very time consuming and expensive.
The problem with removing drilling mud and other solids from surface well equipment is twofold; (1) the thixotropic mud stiffens as a gel to hold solids in the non-Newtonian state, and (2) diluting the mud with water reduces its viscosity and gel formation whereby mud and other solids readily deposit in the lower parts of flow channels and in stagnant regions of the system. Not all parts of the surface well system are exposed for cleaning as are the several mud tanks, shakers, manifolds, etc. For example, there are literally several hundred feet of interconnecting piping, valves, surge and pulsations control devices and the like that cannot be cleaned mechanically. Importantly, the pumping system includes piping of over one foot in diameter. For these reasons, simple mechanical cleaning with clean water washing cannot leave the surface mud equipment scrupulously clean so that a completion brine is not contaminated by the residual solids it entrains with great affinity. During the cleaning of the mud system, the rig must be shutdown for between 5 and 13 hours on the average. The costs of cleaning ranges from about $3000 to $8000 per hour. Thus, a fast efficient cleaning procedure would save the majority of rig down time in an amount of about $40,000.
The present improved process substantially eliminates all of the drilling mud, including solids such as barite, bentonite, cement, etc. and oil materials, from the surface well equipment prior to entry therein of the solids-free completion and packer brine. As a result, the brine remains substantially free of solids before pumped into the well bore. If brine filtration is needed, the filter loading remains very small and does not involve any significant expense in rig time. The overall rig down time to practice the present process requires only two-to-three hours. This is an important advantage in offshore drilling platforms.